1. Field of the Invention
This invention relates to a novel solvent system for enhancing the solubility of pharmaceutical agents by partial ionization to produce highly concentrated primarily non-aqueous water miscible solutions of those agents; which as liquids are suitable for encapsulation in both softgels (previously known as soft elastic gelatin capsules) and in two piece hard gelatin shells, which can be sealed to retain liquid; which as semi-solids are suitable for encapsulation in two-piece hardshell capsules; and which as solid solutions are suitable for conversion into tablets. The solvent system of the present invention is useful in that it provides for the encapsulation of a pharmaceutical agent in a volume of solution that is small enough to permit easy swallowing. It further provides for the preparation of highly concentrated solutions of a pharmaceutical agent having utility for pour on and spot on preparations in veterinary medicine.
Filled one-piece softgels have been widely known and used for many years and for a variety of purposes. Because softgels have properties which are quite different from telescoping two-piece hardshell capsules, the softgels are capable of retaining a liquid fill material. The fill material may vary from industrial adhesives to bath oils. More commonly, the softgels are used to enclose or contain consumable materials such as vitamins and pharmaceuticals in a liquid vehicle or carrier.
Generally, not all liquids are suitable as vehicles or carriers for enclosing softgels. For example, water, propylene glycol, glycerin and low molecular alcohols, ketones, acids, amines and esters cannot be filled in softgels by themselves and can only be present in small amounts. In particular, concentrations of water greater than 20% will dissolve the gelatin shell. Liquids that are suitable for filling softgels vary from water immiscible liquids such as vegetable oils, aromatic oils, aromatic and aliphatic hydrocarbons, chlorinated hydrocarbons, ethers and esters, to water miscible nonvolatile liquids, such as polyethylene glycols and nonionic surfactants.
There are specified limitations to the use of certain liquids as fill vehicles for softgels. For example, the pH of the fill liquid should not be below 2.5 or above 7.5. At pH's below 2.5, the gelatin is hydrolyzed causing leaking, whereas at pH's greater than 7.5, the gelatin is tanned resulting in decreased solubility of the gelatin shell. Moreover, emulsions of oil/water or water/oil are not suitable for softgel encapsulation because they eventually break up releasing water which dissolves the gelatin shell.
Vitamins and pharmaceuticals that naturally occur as liquids are ideally suited for softgels. These naturally occurring liquids are simply mixed with a miscible liquid carrier which is also suited as a softgel fill.
Vitamins and pharmaceuticals that naturally occur as solids may be filled into softgels in liquid form under primarily one of two approaches--either as a suspension of the solid in a liquid carrier or as a solution of the pharmaceutical agent in the appropriate solvent. Each approach has its attendant problems. For example, in the suspension, the solids must have a particle size no greater than 80 mesh. Coarser materials prevent the softgel filling equipment from functioning properly. They also prevent the achievement of a good "content uniformity" throughout the batch.
By contrast, a solution provides the best liquid form to obtain optimal "content uniformity" in a batch. In addition, a solution provides a faster and more uniform absorption of a pharmaceutical agent than does a suspension. Because of these distinct technical advantages, the solution is preferred over the suspension.
However, a problem in the art is that an appropriate solution of the pharmaceutical agent cannot always be achieved. One constraint is size. Often, it is not possible to dissolve the pharmaceutical agent in a volume of solvent small enough to produce a softgel that is appropriate from the standpoint of economics and patient acceptance. Another constraint, is the solvent itself. The solvent must have sufficient solvating power to dissolve a large amount of the pharmaceutical agent to produce a highly concentrated solution, and yet not hydrolyze, dissolve, or tan the softgel.
It is a primary object of the present invention to provide a solvent system which is capable of producing highly concentrated solutions of pharmaceutical agents and that these highly concentrated solutions be suitable for filling into softgels.
Like the one-piece softgels, the two-piece telescoping hardshell capsules have also been used for many years and for a variety of purposes. Unlike the one-piece softgels, the two piece capsules are not a sealed system and hence are generally not suited for handling liquids. However, a two-piece capsule can handle a liquid without leaking provided that it is properly sealed or that the liquid is converted into a solution which is either solid or semi-solid at room temperature. If the solid or semi-solid solution contained within the two-piece capsule is a highly concentrated solution of a pharmaceutical agent, then the advantages possessed by a solution over a suspension are made available to both the user and the manufacturer. Specifically, the advantage of a faster and more uniform absorption of the pharmaceutical agent is available to the user of the two-piece capsule, while the advantage of uniformity of the batch is available to the capsule manufacturer.
It is a further object of the present invention to provide a solvent system that is capable of producing highly concentrated solutions of pharmaceutical agents that are solid or semi-solid solutions at room temperature and that these solutions also be suitable for two-piece hardshell encapsulation. The highly concentrated solutions that are solid at room temperature have the additional utility of being suitable for conversion into tablets.
Because most pharmaceutical agents are acidic, basic, or amphoteric in nature, it is a further object of this invention to provide a solvent system (pharmaceutical carrier system) which with minor modification could be equally useful for a pharmaceutical agent regardless of its basic, acidic, or amphoteric nature.
Producing a highly concentrated solution of any acidic amphoteric or basic pharmaceutical agent is useful because it permits the encapsulation of a unit dose of the pharmaceutical agent in a softgel or two-piece capsule that is small enough to permit easy swallowing. Filling of a unit dose in a small softgel or 2-piece capsule to permit easy swallowing is useful because it increases patient acceptance of the medication. Patient acceptance is especially important in the case of prescription medications, because patient acceptance of the medication is a substantial step towards solving one of the major problems of prescription drug therapy--patient noncompliance with the prescribed regimen. A further utility of the disclosed solvent system is enhancement of bioavailability of the dissolved pharmaceutical agent. Enhanced bioavailability occurs as a result of delivering the pharmaceutical agent already in solution at the site of absorption, permitting a faster and more uniform absorption to occur.
2. Description of the Related Art
Weber and Molenaar U.S. Pat. No. 3,557,280, teaches the preparation of aqueous solutions of oxytetracycline suitable for intramuscular and intravenous injection or for administration as a syrup in pediatric cases. The Weber and Molenaar (Weber) invention consists of dissolving oxytetracycline in water, in which a given quantity of polyvinylpyrrolidone has been dissolved, to which has been added a suitable quantity of magnesium salt, the pH of which has been adjusted to between 8.0-9.5 using a base such as sodium hydroxide, ammonia, etc.
Although Weber uses polyvinylpyrrolidone to enhance the solubility of oxytetracycline in an aqueous system, Weber neither teaches nor suggests that polyvinylpyrrolidone would also be useful for enhancing solubility of other pharmaceutical agents in non-aqueous systems. Moreover, the aqueous solutions taught by Weber are totally unsuited for either softgel or two-piece encapsulation, since such aqueous solutions would dissolve the gelatin shells.
The present invention differs from Weber in a number of other respects as well. Whereas Weber teaches the formation of relatively dilute solutions (1-20%), the present invention teaches the preparation of more highly concentrated solutions (30-80%) requiring more skill than that disclosed in Weber.
Whereas Weber teaches at column 3, line 25 that a salt suitable for chelating, such as magnesium, is "essential" to enhance the solubility of oxytetracycline, the present invention teaches the preparation of highly concentrated non-aqueous solutions of pharmaceutical agents without resorting to chelate formation to enhance solubility.
Although both Weber and the present invention use the base, sodium hydroxide, in the preparation of their pharmaceutical formulations, the role played by the sodium hydroxide in each invention differs. Whereas Weber uses the sodium hydroxide to adjust the pH to between 8.0-9.5 as to increase the shelf life of the oxytetracycline solution (see Merck Index, 9th edition at p. 904), the present invention uses the sodium hydroxide to enhance the solubility of an acidic pharmaceutical agent by forming as much of the ionized form of the acidic agent as is capable of being solvated by the system.
Gardella et al, U.S. Pat. No. 4,002,718, teaches at column 3, line 47, the use of small amounts of polyvinylpyrrolidone or glycerin to hasten dissolution of micronized digoxin in a liquid vehicle, polyethylene glycol, to form a solution suitable for softgels. Unlike Gardella, which only teaches the use of polyvinylpyrrolidone or glycerin as a formulatory agent to hasten or speed up dissolution, the present invention teaches the use of glycerin or polyvinylpyrrolidone to enhance or increase the amount of the pharmaceutical agent that is soluble in a given volume of liquid.
In further contrast, Gardella only teaches the production of very dilute solutions (0.1%) for encapsulation, whereas the present invention teaches the production of highly concentrated solutions (30-80%). Because the solutions of the present invention are 300-800 times more concentrated than that taught by Gardella, the teachings of Gardella are not applicable to the present invention. Moreover, Gardella does not even suggest that glycerin, propylene glycol, or polyvinylpyrrolidone would be useful for enhancing solubilities of pharmaceutical agents as to produce 30-80% solutions of those agents.
Wagner, U.S. Pat. No. 4,562,192, suggests the use of polyvinylpyrrolidone as a formulatory agent (adjuvant) for pharmaceutical preparations. However, Wagner neither teaches nor suggests that polyvinylpyrrolidone is useful for enhancing the solubility of a pharmaceutical agent in a given volume of liquid.